The present invention relates to a process for the production of polyurethane sandwich elements (so-called panels) in which the polyurethane reaction mixture is applied both to the upper and to the lower outer layer, and then the outer layers which are wetted with polyurethane reaction mixture are brought into contact with one another. This invention also relates to an apparatus for the production of these polyurethane sandwich elements (i.e. panels).
Polyurethane sandwich elements (i.e. panels) are, in general, produced by a continuous process. In this process, the panels are produced as a continuous product on so-called Contimats in thicknesses of, as a rule, approx. 20 to 200 mm. However, thicknesses of less than 20 mm and of more than 200 mm are also possible. Such a Contimat in this context conventionally comprises a circulating upper belt for guiding the upper outer layer and a circulating lower belt for guiding the lower outer layer, a feed device for the upper outer layer, a feed device for the lower outer layer, a shaping zone within which the polyurethane reaction mixture foams and reacts between the upper outer layer and the lower outer layer, a length-cutting device for the panel produced, and a metering station with a mixing head for application of the polyurethane reaction mixture to the lower outer layer. A Contimat according to the prior art is shown in FIG. 1.
In the case of panels with sheet metal outer layers, a polyurethane layer with a bulk density of 45±5 kg/m3 is conventionally present between the upper and the lower outer layer. However, it is desirable in this context to establish the lowest possible bulk densities of, for example, 40 kg/m3 or, for example, of even only 38 kg/m3, in order to minimize the amount of starting substances consumed.
For various reasons, however, technical problems arise here. On the one hand, the complete filling of the canted edge zones of sheet metal outer layers is inadequate at a low bulk density. On the other hand, in the case of thin panels, such as, for example 20 mm thick, the distribution of the polyurethane reaction mixture is not uniform at a low bulk density. This is particularly a problem at a relatively high belt speed of, for example, above 30 m/min. In both cases, panels of lesser quality or waste consequently result. A further problem is the low adhesion between the polyurethane foam and the upper sheet metal outer layer at low bulk densities. This, in turn, can lead to panels of lesser quality or waste.
To avoid problems such as these, the panels are typically produced with increased bulk densities of conventionally 45 to 50 kg/m3, i.e. the so-called “overpacking” technique. In “overpacking”, the material flow of polyol, isocyanate, and blowing agent, and thus of polyurethane foam are increased, and the foaming pressure is also increased. This results in the adequate filling of the canted edge zones. The increased foaming pressure results here from the increased amounts of polyurethane reaction mixture and blowing agent, which leads to foaming of the polyurethane foam, and therefore to a build up of pressure in the closed-off space between the upper outer layer and the lower outer layer in the shaping zone. At a higher material flow, the distribution of the polyurethane reaction mixture on the lower outer layer is, of course, also better. The adhesion of the polyurethane reaction mixture to the upper sheet metal outer layer is likewise improved by the higher material flow, but above all by the higher foaming pressure.